The insulin receptor of cultured human lymphocytes has been characterized by biosynthetic labeling with radioactive amino acids (methionine and leucine) and sugars (fucose, galactose, glucosamine and mannose). In addition the subunits of the receptor have been labeled by cell surface methodology that reveal proteins (lactoperoxidase/Na125I) or glycoproteins (galactose oxidase/NaBH4, periodate/NaBH4). The receptor has, as shown by all these methods, two major subunits of molecular weights 135,000 and 95,000 and a minor component of 205,000-210,000. The two major subunits are also revealed by affinity cross-linking with 125I-insulin. The interaction of the solubilized insulin receptor with twelve different lectins have also been analyzed. Using these labeling techniques we have measured directly the turnover rate of the receptor subunit. We have found that the half life of both major insulin subunits is 9-12 hours. When the cells are cultured in medium containing insulin, the degradation rate is accelerated over 3-fold accounting for the decrease in receptor concentration observed in down-regulation.